The main process in fabrication of silicon chips is photolithography. Photolithography is a process of imaging a desired pattern on a silicon wafer coated with a photoresist material. This is achieved using a quartz plate (mask) on which a desired pattern is printed. A light (typically a 193 nm laser or an Extreme Ultra Violet laser) is projected through the mask traveling through specialized optics that images the desired pattern at wafer plane on the photoresist. Each mask used is designed in such a way that slight changes in photolithography conditions (typically laser dose and defocus) still provide the desired pattern up to a certain tolerance. The extent of defocus and dose change that still provide the desired pattern up to a specified tolerance is referred to as process window.
During the photolithography process in the wafer fabrication facilities (fabs) following issues may occur:                a. The actual process window of the mask is less than what it was designed for (mask fabrication issue, design error, OPC modeling error, etc.).        b. After a certain number of mask exposures (to laser) mask goes through physical change that decrease the process window size. This can be due to oxidation, Crystal growth, pellicle degradation, etc.        c. When a mask returns from cleaning to production, the process window is decreased. (cleaning induced degradation)        
As a result of this reduction in process window certain layout segments may be printed in an erroneous way which may in turn lead to device failure.
The main challenge is in identifying all locations with high risk of erroneous wafer print.